You know, you could use the "wilma9" with 1x600w depending upon what style of reflector you choose, they are 3pots X 3pots and i think they are 11litre pots. So you could realistically grow slightly smaller plants but more numbers.
I think with the WILMABIG4 you could run into the same problem as the normal 4 where the plants grow into one another. They are 18litre pots in the BIG4 but it could quite easily happen just like the normal 4.
J